<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 300px;
            height: 300px;
        }
    </style>
</head>

<body>
    <div>

    </div>
    <script>
        function col(flag) {
            let arr = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f']
            let str = '#'
            if (flag) {
                for (let i = 0; i < 6; i++) {
                    let ran = Math.floor(Math.random() * arr.length)
                    str += arr[ran]
                }
                return str
            } else {
                let r = Math.floor(Math.random() * 256)
                let g = Math.floor(Math.random() * 256)
                let b = Math.floor(Math.random() * 256)
                return `rgb(${r},${g},${b})`
            }
        }
        console.log(col(true))
        console.log(col(false))

        const divv = document.querySelector('div')
        divv.style.backgroundColor = col(true)

    </script>
</body>

</html>